The motion of a spacecraft can be divided into two types of motion. First, the center of mass of the spacecraft may be moving relative to the center of the Earth, and second the spacecraft may be moving about its own center of mass.
Control of the position and velocity of a spacecraft relative to the center of the Earth are the subject of celestial mechanics, or space navigation and the present invention does not deal with controlling this type of motion.
The orientation and motion of a spacecraft about its center of mass are referred to as the attitude and attitude motion of the spacecraft. In general the attitude and attitude motion of a spacecraft are specified by reference to three orthogonal coordinates called roll, pitch and yaw.
There are a variety of known techniques for controlling the roll, pitch and yaw of a spacecraft. The known techniques can be divided into "passive" stabilization techniques which require no power input and "active" stabilization techniques which require a power source.
The known techniques for passive stabilization include techniques such as "gravity gradient" stabilization, "spin" stabilization", "solar radiation" stabilization, "aerodynamic" stabilization and "permanent magnet" stabilization. The known techniques for active stabilization include "gas jet" stabilization, "electromagnet" stabilization, "reaction wheel" stabilization, and "ion thruster" stabilization. Each of the active techniques use or consume power during their operation.
A variety of known active and passive techniques for controlling the attitude of a spacecraft are explained in a book entitled Spacecraft Attitude Determination and Control Edited by James R. Wertz and published by D. Reidel Publishing Company, 1978, 1985.
Passive stabilization of a spacecraft using a gravity-gradient boom is attractive due to the simplicity of the apparatus used, however, spacecraft stabilized by a gravity-gradient boom often exhibits librational motion due to errors in the initial attitude acquisition process. Passive dampers such as spring dampers, hysteresis dampers, and eddy current dampers can be used to damp librational motion; however, such dampers have certain shortcomings.
Spring dampers are more effective in removing librations in the orbit plane than in removing librations perpendicular to the orbit plane. Eddy current dampers move a spacecraft into a nonzero bias attitude, resulting in limit cycle oscillations. Furthermore, eddy current dampers provide relatively weak damping and they have the disadvantage that the devices include mechanically moving parts.
For spacecraft which have relatively equal pitch and roll moment of inertia, a gravity gradient boom together with a momentum wheel to impart sufficient stability around the yaw axis have been used to obtain three axis control.
It is known that three orthogonal electromagnets controlled by the output of a magnetometer can be used in the acquisition phase of the stabilization process. In such system after the space craft is grossly stabilized using the orthogonal electromagnets, one or more reaction wheels or momentum wheels are normally used to control the pitch, roll and yaw to the desired degree of precision. Such systems in effect have two separate attitude control systems, one of which is used during the acquisition phase of the operation and one of which is used during the remaining part of the flight.